The present invention relates to conformal doped aluminum coatings on a patterned substrate and a methodology and apparatus to prepare such doped coatings. More particularly, the present invention is directed to the controlled, reproducible growth by thermal or plasma-assisted CVD (PACVD) processes of ultrathin Cu layers which are subsequently used as seed surface for the in-situ thermal or plasma assisted chemical vapor deposition of smooth, void-free, and dense copper-doped aluminum films which conformally coat semiconductor device substrates with patterned holes, vias, and trenches with aggressive aspect ratios (hole depth/hole width ratios).
The ever increasing demand for higher density and enhanced performance in deep sub-quarter micron integrated chip (IC) device technologies is placing enormous pressure on intrachip interconnect architecture development. Predictions published in the Semiconductor Industry Association Technology Roadmap for Semiconductors, See The International Technology Roadmap for Semiconductors, 1999 Edition (Semiconductor Industry Association, San Jose, Calif., 1999), indicate that the emerging needs of advanced logic and microprocessor systems could require over ten levels of interconnects. One of the key problems in the generation of these multiple conductor levels involves the fabrication of well-defined and precisely-patterned vertical electrical connections (vias) between different interconnect planes of the chip.
As the traditional building block of the IC interconnects, Al alloys have played a major role in the evolution of the computer age. When alloyed with 0.5 wt % copper, Al exhibits enhanced electromigration resistance while maintaining good electrical conductivity. In addition to its ability for self passivation in air and ease of patternability in chlorine based plasmas, Al bonds well to SiO2 and diffusion barriers of titanium nitride and titanium. See S. P. Murarka, Metallization: Theory and Practice for VLSI and ULSI (Butterworth-Heinemann, Boston, 1993). In light of these properties, Al based metallizations are predicted not only to continue as the interconnection workhorse of the integrated circuit industry in the foreseeable future, but will extend their role in providing contact and via hole plugs for all wiring levels.
Unfortunately, in the past decade, the deposition of Al alloys into small vertical holes cut into interlevel dielectrics has become increasingly problematic as feature sizes decrease below half micron. Poor metal step coverage and the resulting incomplete filling of vias with physical vapor deposited (PVD) Al alloys generated serious process and reliability concerns. Al reflow and other high temperature Al alloy sputtering, or physical vapor deposition (PVD), processes are presently being explored and implemented as potential low cost alternatives which provide conformal via fill and ease in integration in device fabrication process flow. However, the repetitive exposure to high deposition temperatures required in a multilevel metal (MLM) architecture may adversely affect the device during processing. Secondly, the need for high temperature may greatly restrict the implementation of a number of new low dielectric constant materials into future interconnect architectures. The high temperature excursions could also result in barrier failure, which would be problematic at the contact level and lead to undesirable levels of junction leakage. See P. Singer, Semiconductor International 17 (1994) 57.
Chemical vapor deposition (CVD) of Al presents a viable alternative to PVD due to its inherent ability to grow films conformally on via and trench structures. Efforts to develop CVD Al deposition techniques date as far back as the late 1940s, wherein a variety of chemical sources were used which included Al halide, alkyl, and organometallic sources. See, for example, C. F. Powell, J. H. Oxley, and J. M. Blocher, Jr., Vapor Deposition (Wiley, New York, N.Y., 1966) p. 27; and H. J. Cooke, R. A. Heinecke, R. C. Stern, and J. W. C. Maas, Solid State Technol. 25 (1982) 62. The resulting Al films, regardless of the chemical source used, exhibited extensive surface roughness, high resistivity, and substantial contamination. Attempts to re-investigate these systems for device applications were revived in the mid-1980s. In these cases, evaluations were performed of Al CVD films generated from metal-organic sources such as tri-isobutyl aluminum (TIBA). See, for example, B. E. Bent, R. G. Nuzzo, and L. H. Dubois, J. Am. Chem. Soc. 111 (1989) 1634, H. O. Pierson, Thin Solid Films, 45 (1977) 257; and R. A. Levy, P. K. Gallagher, R. Contolini, and F. Schrey, J. Electrochem. Soc. 132 (1985) 457. In these initial batch process type studies, which used hot wall CVD reactors, carbon contamination, surface roughness, and low deposition rates posed unacceptable process problems. These unsuccessful experiments were prematurely abandoned in favor of more mature and manufacturable CVD tungsten metallization processes.
Tungsten CVD use could probably continue below the 0.1 xcexcm device technology, as equipment suppliers focus their efforts on enhancing throughput, reducing particles, and improving cost-of-ownership. See P. Singer, Semiconductor International 17 (1994) 57. However, the cost associated with the deposition and etchback of CVD tungsten is substantially higher than of its Al alloy counterpart. Additionally, the use of Al alloy plug, with its approximately 100% lower electrical resistance (R) than its tungsten counterpart, provides the promise of substantially reduced RC delay, where C is the capacitance of the insulator. These expectations have revived interest in the development of a low temperature CVD based process for Al/0.5 at % Cu that is capable of filling small, high aspect ratio holes that are patterned in thermally fragile plastic-like, low dielectric constant organic polymers. Most recent CVD aluminum work has focused on the use of metal-organic precursors of the type diethylmethylaluminum alane or DMEM and dimethylaluminum hydride or DMAH. See, for instance, M. E. Gross, K. P. Cheung, C. G. Fleming, J. Kovalchick, and L. A. Heimbrok, J. Vac. Sci. Technolo. A9 (1991) 1; M. E. Gross, L. H. Dubois, R. G. Nuzzo, and K. P. Cheung, Mat. Res. Symp. Proc., Vol 204 (MRS, Pittsburgh, Pa., 1991) p. 383; W. L. Gladfelter, D. C. Boyd, and K. F. Jensen, Chemistry of Mater. 1 1989) 339; D. B. Beach, S. E. Blum, and F. K. LeGoues, J. Vac. Sci. Technol. A 1989) 3117. The molecular structure of these precursors is distinguished by the presence of aluminum-hydrogen (i.e. Alxe2x80x94H) bonds. This feature provided a clean chemical pathway to eliminate the precursor""s hydrocarbon groups at relatively low temperatures to yield pure aluminum films. More specifically, the AlH3 groups in DMEAA, and the Alxe2x80x94H groups in DMAH permitted pure aluminum film growth at temperatures as low as, respectively, 100xc2x0 C. and 200xc2x0 C. This feature made these compounds the candidates of choice in most on-going Al CVD development activities, and led to the successful growth of device-quality aluminum with conformal step coverage for substrates having aggressive holes and trenches (i.e., with a diameter of 0.25 xcexcm or smaller) and high aspect ratios (i.e., the ratio of hole depth to hole width equal to or greater than about 4:1).
In spite of the recent success of both PVD processing, such as Al reflow, and CVD processing, both thermal and plasma assisted, at the formation of device-quality Al thin films, there exists a critical need for a processing technology to provide doped aluminum films (aluminum with a few percent of other elements, such as copper, carbon, tungsten, tantalum, titanium, palladium, gold, silver, platinum, silicon, germanium, samarium, zirconium, palladium, magnesium, etc.) suitable for ULSI fabrication. Copper doping is needed to significantly enhance aluminum""s resistance to electromigation and allow aluminum interconnects to sustain the high current densities ( greater than 106 A/cm2) required for proper operation of the IC devices. The Cu doped Al films must be of especially ultra high quality, in terms of purity, with impurity concentrations well below 1 atomic percent, must exhibit excellent electromigration properties, must be highly specular, with extremely smooth surface morphology, and must be conformal to the complex topography of ULSI circuitry to provide complete filling of aggressive via and trench structures.
In this respect, Dyer et al. (U.S. Pat. No. 5,273,775) disclose a chemical vapor deposition process for the selective growth of Alxe2x80x94Cu alloys by selectively depositing a copper layer on the conducting regions of a patterned silicon substrate and then depositing an aluminum layer over the copper layer. The copper and aluminum layers were annealed to form an aluminum copper alloy film. It was necessary to limit the temperature for the aluminum deposition to below 180xc2x0 C. to achieve selective Al growth. Loss of selectivity was observed even when the substrate temperature was only slightly increased 185xc2x0 C. This temperature limitation prevented the in-situ formation of copper-doped aluminum, and necessitated the introduction of an annealing step subsequently to the deposition of the Cu and Al layers to ensure the growth of copper-doped aluminum. In addition, it is known in the art that the formation of large aluminum grains during a low temperature deposition can lead to a porous, highly-resistive, aluminum film. Therefore, there is a need for a process that allows the preparation of copper-doped aluminum films in-situ, i.e., where copper diffuses through the aluminum layer as it is being formed, and without the need for subsequent annealing to drive copper into the aluminum layer and completely alloy the copper and aluminum layers to form a homogeneous copper-doped aluminum film.
It is also highly desirable that the copper and aluminum deposition steps be performed without any intermediate exposure to air due to the high affinity of aluminum to oxygen. Oxygen contamination would result in oxidized, highly resistive, aluminum films which are not appropriate for computer chip interconnect applications. To avoid undesirable air exposure and associated film oxidation, the deposition of the copper and aluminum layers must be carried out in the same reaction chamber, i.e., without the necessity of transferring a substrate coated with a single film (Al or Cu) to another reaction chamber to deposit the other film. Alternatively, deposition of the Al and Cu layers could be performed sequentially in two separate reaction chambers that are connected by a vacuum tight substrate handling system or load lock that allows substrate transfer between the individual reaction chambers without exposure to air.
Other workers have successfully produced in-situ copper doped aluminum films. Aluminum films doped with 0.7-1.4 wt % copper were grown through the simultaneous decomposition in the same CVD reactor of dimethylaluminum hydride (DMAH) and cyclopentadienyl copper triethylphosphine which were employed, respectively, as the aluminum and copper sources. See T. Katagiri, E. Kondoh, N. Takeyasu, T. Nakano, H. Yamamoto, and T. Ohta, Jpn. J. Appl. Phys. 32 (1993) L1078. Unfortunately, the copper source used in the work was highly reactive and unstable during transport and handling, which makes it undesirable for real industrial applications. Clearly, there is a critical need for stable copper sources which are free of oxygen, fluorine, and halides, and which are compatible with aluminum precursors to prevent any cross-contamination effects during film growth. The unavailability of copper source precursor with these properties target specifications stated above necessitates the development of an in-situ deposition of sequential bilayers of Al and Cu.
In this respect, it is extremely desirable that a process be developed which can control the deposition of the Cu layers down to extremely low thicknesses, e.g., below 2 nm. This feature is required to ensure that, upon annealing or mixing with aluminum, copper concentration in the doped aluminum film does not exceed 0.5 wt %. This upper limit is needed to prevent any problems in the subsequent Al interconnect etching and patterning steps, although recent technological advances in these areas could eventually push that number to values as high as 3 to 5 wt %. The process must allow deposition of a bilayered stack consisting of an ultrathin Cu layer followed by Al. The latter is particularly desired because copper is known to inhibit undesirable aluminum grain growth through alloying. Additionally, copper""s use as growth surface in aluminum CVD provides a good seed layer for the uniform nucleation of aluminum grains, leading to smaller grain size and significantly smoother morphology, as desired in microelectronics applications.
In addition, the process must be flexible to allow use of thermal CVD processing for ultrathin copper films in combination with plasma assisted CVD of aluminum or vice versa. In this respect, it might be desirable in some instances to use PVD techniques, such as sputtering, reflow, plating, or electroplating, in combination with, or in lieu of, the CVD approaches described above to deposit one or both metallic layers. Similarly, substrate bias could be applied to the substrate during either deposition step. The purpose is to form a xe2x80x9csoftxe2x80x9d plasma region above the substrate to assist in the actual decomposition process, and/or attract ionized aluminum or copper ions to the various topographical regions of the substrate, leading to more conformal via and trench filling
The invention includes a method and apparatus for the chemical vapor deposition of conformal copper-doped aluminum layers on substrates. Primarily, the invention deposits copper-doped aluminum metallization layers on semiconductor substrates, such as silicon and gallium arsenide. The invention deposits the two metals in-situ, sequentially, with ultrathin copper layer being deposited first and used as seed layer in the deposition of smooth aluminum films with the grain size and texture required for microelectronics applications. With this invention, aluminum films are deposited onto the copper layer in a manner that allows the use of this layer as a xe2x80x9creservoirxe2x80x9d or supply of copper atoms that interact with the aluminum film as it is growing to inhibit the formation of void-rich, low-density, high-resistivity aluminum films with large grain size and high surface roughness. According to this invention, the aluminum film is grown on the copper seed layer at a temperature higher than 185xc2x0 C. to initiate interaction between the copper and aluminum while the Al film is growing. This characteristic is essential to drive the partial diffusion of copper through the CVD Al layer as it is growing to ensure that Cu is inhibiting the nucleation of large aluminum grains. Otherwise, the formation of large aluminum grains leads to a porous, highly-resistive, aluminum film that is not usable in microelectronics interconnect applications.
Also in accordance with this invention, sequentially deposited copper-aluminum layers could then be annealed in-situ or ex-situ to complete the formation of a completely homogeneous copper-doped aluminum film. The Cu, Al, and in-situ annealing could take place either in the same reactor, in two separate reactors, one used for deposition and the other for annealing, or in three separate reactors. In the cases when more than one reactor is used, it is preferred that the reactors be inter-connected through leak-tight transfer arms/load locks. The transfer arms/load locks allow sample transfer between the different reactors without exposure to air. Alternatively, the invention provides for depositing the aluminum film on the copper seed layer at a temperature sufficient to form a smooth, copper-doped, aluminum film without the need for the annealing step.
The invention provides a means to accurately and repeatably transport copper precursor gas to the reaction zone at a rate and flux which allow reproducible deposition of ultrathin copper layers, e.g., as thin as 1.0-1.5 nm, within 0.1 to 0.2 nm accuracy. The invention uses copper source precursors which have been diluted in a precursor carrier medium at significantly reduced precursor concentration levels. With the invention, any suitable copper precursor can be used, regardless of whether it is a solid, liquid or gas. For solid or liquid copper source precursors, the precursor carrier medium could include solvents, water, or an adducted form of the source precursor. In this case, the mixture of copper precursor and associated medium can be vaporized prior to its introduction to the reaction zone, or inside the reaction zone. In the case of gaseous copper source precursors, the precursor carrier medium could include inert gases such as nitrogen, argon, xenon, or helium, or more active gases such as hydrogen. The invention uses highly diluted precursor concentrations to enable tight control over precursor transport and delivery rates, and to ensure the delivery of very low fluxes of copper precursor gas to the reaction zone. Substantially, any precursor delivery system, including pressure-based bubblers, liquid delivery systems, direct liquid injection systems, standard and hot source mass flow controllers, can be used to deliver the mixtures of precursor and carrier medium to the reaction zone.
The invention also provides a heat assisted chemical vapor deposition process and apparatus. With the invention, copper then aluminum are deposited sequentially on the substrate. This process is carried out using copper and aluminum precursor gases in combination with the precursor transport means discussed above. As in the case of the copper precursors, the aluminum precursors could be solid, liquid, or gaseous at room temperature, and could be heated to transform them into a gas which is easily transportable to the reactor. For both copper and aluminum, the precursors gases are transported with suitable carrier gases such as hydrogen, argon, nitrogen, or a mixture thereof. Deposition takes place in a reactor at pressures ranging from one atmosphere to high vacuum and at temperatures above 185xc2x0 C., in order to facilitate initiate interaction between the copper and aluminum while the Al film is growing. Under these conditions, copper partially diffuses through the Al layer as it is growing and inhibits the nucleation of large aluminum grains. Otherwise, as well known by those who are skilled in the art, the formation of large aluminum grains leads to a porous, highly-resistive, aluminum film that is not usable in microelectronics interconnect applications.
The invention also provides for plasma, light, or laser assisted chemical vapor deposition. It uses the energy provided by the plasma, light, or laser to deposit either the copper or aluminum layer, or both. In these cases, an electrical bias could be applied to the substrate at power densities ranging from 0.005 W/cm2 to 100 W/cm2 and frequencies ranging from 1 Hz to 108 kHz. The local electrical field in the region above and at the surface of the substrate enhances the rate of impingement of copper and aluminum ions from the copper and aluminum precursor gases, leading to more conformal coating of the topographies of microelectronics device structures.
Four key aspects of the invention are: (a) the application of an ultrathin ( less than 10 nm) copper layer as a seed layer in the deposition of smooth, copper-doped, aluminum films. The aluminum films are deposited at temperatures above 185xc2x0 C. to allow copper atoms from the ultrathin seed layer to interact with the growing aluminum film to inhibit undesirable large grain growth and yield texture and composition required for microelectronics applications, (b) the use of mixtures of copper source precursors which have been diluted in a precursor carrier medium at significantly reduced precursor concentration levels to allow accurate and repeatable copper precursor transport to the reaction zone at a rate and flux which allow reproducible deposition of ultrathin copper layers, e.g., as thin as 1-1.5 nm, within 0.1-0.2 nm accuracy, (c) the use of copper source precursors of low volatility, in combination with the approach described under (b) above, in cases when additional, tighter, control on ultrathin layer thickness is required, and (d) the application of a substrate bias, when needed, to form a soft plasma region above the substrate and enhance the rate of impingement of copper and aluminum ions from the copper and aluminum precursor gases, leading to more conformal coating of the topographies of microelectronics device structures.